The long term goals of this research program are to contribute to the chemistry and biochemistry of novel nucleosides, nucleotides and related molecules with useful therapeutic potential against the infectivity of the AIDS viruses, including drug-resistant HIV variants. The specific aims of this proposal are the design, synthesis, enzymology and anti-HIV studies of the phosphorylated analogs of related families of structurally and stereochemically defined compounds referred to as isodideoxynucleosides, for which there is some documentation for strong anti-HIV activity. Rationale for the selection of these compounds for investigation includes, in addition, considerations of phosphorylation including in vivo phosphorylation, cellular stability, bioavailability, selectivity of incorporation into viral DNA and inhibition of HIV reverse transcriptase, and host cell toxicity. The synthetic work will involve development of approaches to optically pure, novel isodideoynucleosides and their conversion to phosphates, methylenephosphonates, H-phosphonates, phosphoramidates and triphosphates including various masked pro-drugs forms. Complete characterization including stereochemistry of the final products will be performed by (31)P, (13)C, (1)H and (15)N NMR spectroscopy, high-resolution FAB mass spectrometry, UV and optical activity data, elemental analysis, and single crystal X-ray data. Stabilities of all target compounds, including the behavior towards selected nucleoside metabolizing enzymes, will be investigated. Collaborative antiviral studies of highly purified forms of the phosphorylated target compounds, their pro-drugs forms, the parent isodideoxynucleosides and their acylated lipophilic derivatives, will be carried out against HIV-1, HIV-2, and AZT-resistant HIV strains. A number of cell lines will be used in these studies including MT2, MT4, ATH8 and H9. Data on inhibition of the cytopathic effect of HIV in MT2, MT4 and ATH8 cell lines, on host cell cytotoxicity, on inhibition of expression of HIV-1 p24 Gag protein in H9 cells, on inhibition of proviral DNA synthesis in peripheral blood mononuclear cells, and on therapeutic indexes will be determined and analyzed. The proposed compounds have a high probability of having useful anti-HIV activity and the investigation will contribute to filling some of the gaps in knowledge in this area.